Liquid suspension of cerium oxide particles

ABSTRACT

The present invention relates to a suspension of cerium oxide particles in a liquid phase, in which said particles comprise secondary particles comprising primary particles, and a process for preparing said liquid suspension in which the cerium IV/total cerium molar ratio before precipitation is comprised between 1/10000 and 1/500000 and that the thermal treatment is being carried out under an inert atmosphere.

The present invention relates to a suspension of cerium oxide particlesin a liquid phase, in which said particles comprise secondary particlescomprising primary particles, and a process for preparing said liquidsuspension in which the cerium IV/total cerium molar ratio beforeprecipitation is comprised between 1/10000 and 1/500000 and that thethermal treatment is being carried out under an inert atmosphere.

PRIOR ART

The following discussion of the prior art is provided to place theinvention in an appropriate technical context and enable the advantagesof it to be more fully understood. It should be appreciated, however,that any discussion of the prior art throughout the specification shouldnot be considered as an express or implied admission that such prior artis widely known or forms part of common general knowledge in the field.

Chemical-mechanical polishing (CMP) slurries are used, for example, toplanarize surfaces during the fabrication of semiconductor chips andrelated electronic components. CMP slurries typically include reactivechemical agents and abrasive particles dispersed in a liquid carrier.The abrasive particles perform a grinding function when pressed againstthe surface being polished using a polishing pad, and separately, thereactive chemical agents serve to oxidize the surface.

The development of the electronics industry requires the increasinglyconsiderable use of compositions for polishing various parts such asdiscs or dielectric compounds. These compositions are in the form ofsuspensions and they must correspond to a certain number ofcharacteristics. For example, they must offer a high degree of removalof material, which reflects their abrasive capacity. They must also havea defectuosity which is as low as possible; the term “defectuosity” isintended to mean in particular the amount of scratches exhibited by thesubstrate once treated with the composition.

It is generally believed that for reasons of stability and of ease ofuse, these suspensions must consist of particles of submicronicdimension, i.e. generally less than 300 nm and that the presence ofparticles that are too fine in these suspensions reduces their abrasivecapacities. Moreover, particles that are too large can contribute to anincrease in the defectuosity as taught by WO2008/043703 publication.There is therefore a need for suspensions in which the particles aremonodisperse. It should also be noted that, in order to obtain optimalperformance levels, this monodispersity should apply both to the primaryparticles and to the secondary particles, i.e. the aggregates consistingof the primary particles.

It is thus understood that the development of these suspensions is acomplex problem.

INVENTION

This application claims priority to European application No. 13306732.2,the whole content of this application being incorporated herein byreference for all purposes.

Should the disclosure of any patents, patent applications, andpublications which are incorporated herein by reference conflict withthe description of the present application to the extent that it mayrender a term unclear, the present description shall take precedence.

The object of the invention is to provide suspensions of cerium oxideparticles providing a sufficient high removal rate and as well as a goodplanarization in the polishing application while avoiding an increase inthe defectuosity in order to reduce the amount of scratches exhibited bythe substrate once treated with the composition.

It appears indeed, contrarily to the knowledge of the prior art, that asuspension providing secondary and primary particles of generallyconsidered high sizes will not lead to attended drawbacks in thepolishing application, notably with the proviso to follow specificparameters relative to sizes and standard deviations of said sizes.

The present invention concerns then a suspension of cerium oxideparticles in a liquid phase, in which said particles comprise secondaryparticles comprising primary particles, wherein:

-   -   said secondary particles have an average size D50 comprised        between 105 and 1000 nm, with a standard deviation comprised        between 10 and 50% of the value of said average size of said        secondary particles; and    -   said primary particles have an average size D50 comprised        between 100 and 300 nm, with a standard deviation comprised        between 10 and 30% of the value of said average size of said        primary particles.

The present invention also concerns a process for preparing a ceriumoxide suspension, notably as previously defined, comprising at least thefollowing steps:

(a) treating a solution comprising at least a cerium III salt, a ceriumIV salt and a base, under an inert atmosphere, whereby a precipitate isobtained; the cerium IV/total cerium molar ratio before precipitation iscomprised between 1/10000 and 1/500000;

(b) subjecting the medium obtained in the preceding step to a thermaltreatment under an inert atmosphere, at least one of the steps (a) or(b) being carried out in the presence of nitrate ions; and

(c) acidifying and washing the medium thus obtained, whereby thesuspension is obtained.

Other characteristics, details and advantages of the invention willemerge even more fully upon reading the description which follows, thevarious concrete but non-limiting examples intended to illustrate it.

DEFINITIONS

Throughout the description, including the claims, the term “comprisingone” should be understood as being synonymous with the term “comprisingat least one”, unless otherwise specified, and “between” should beunderstood as being inclusive of the limits.

For the remainder of the description, the expression “suspension ofcerium oxide particles” denotes a system consisting of solid fineparticles of submicronic dimension based on this oxide, stably dispersedin a liquid phase, it being possible for said particles to alsooptionally contain residual amounts of bound or adsorbed ions such as,for example, nitrates or ammoniums.

Still for the remainder of the description, the term “specific surfacearea” is intended to mean the B.E.T. specific surface area determined bynitrogen adsorption in accordance with standard ASTM D 3663-78established based on the Brunauer-Emmett-Teller method described in theperiodical “The Journal of the American Chemical Society, 60, 309(1938)”.

The particles of the suspension are based on cerium oxide which isgenerally crystalline ceric oxide.

Secondary particles are aggregates aggregated from other, finerparticles, subsequently called primary particles.

The average value of the size of the primary particles is determined byTEM micrograph, considering that crystal structure may be confirmed byXRD.

The standard deviation mentioned in the present invention has the usualmathematical meaning, it is the square root of the variance and it isnotably expressed in WO2008/043703 publication.

For the entire description regarding the secondary particles, theaverage size and the dispersion index are the values obtained byimplementing the laser diffraction technique using a laser particlesizer (distribution by volume).

The term “dispersion index” is defined in WO2008/043703 publication.

The term “dispersion index” is intended to mean the ratio:

σ/m=(d90−d10)2d50

in which:

d90 is the particle size or diameter for which 90% of the particles havea diameter of less than d90;

d10 is the particle size or diameter for which 10% of the particles havea diameter of less than d10;

d50 is the average size or diameter of the particles.

DETAILS OF THE INVENTION

Object of the present invention then comprises a suspension of ceriumoxide particles in a liquid phase, in which said particles comprise atleast secondary particles comprising at least primary particles.Suspension of the present invention may comprise secondary particles andprimary particles.

Notably without a specific deagglomeration step, secondary particles mayhave an average size D50 comprised between 105 and 1000 nm, preferablybetween 110 and 800 nm, more preferably between 110 and 700 nm.

Several deagglomeration of particles may be carried out, such as forexample double impact jet treatment or ultrasonic deagglomeration. Inthis case second particles having an average size D50 preferablycomprised between 105 and 300 nm, more preferably between 110 and 250nm, again more preferably between 110 and 200 nm.

More details may be given for instance regarding the double impact jettreatment without limiting the content of the present invention. Thesecondary particle size may be controlled by the pressure of slurry inthe treatment machine in case of the double impact jet treatment, as thevelocity of slurry from the nozzle in the machine is usually decided bythe pressure. The secondary particle size depends on the pressure ofslurry and the pass number wherein the deagglomeration by double impactjet treatment is repeated at the same pressure.

Secondary particles have an average size D50 with a standard deviationcomprised between 10 and 50% of the value of said average size of saidsecondary particles, preferably between 15 and 45%.

Moreover, according to another advantageous characteristic of theinvention, these secondary particles are themselves also monodisperse.They may have a dispersion index of at most 0.6. This index can be inparticular of at most 0.5, more particularly at most 0.4. Preferably,the dispersion index of secondary particles is comprised between 0.22and 0.6, more preferably between 0.26 and 0.6, particularly between 0.26and 0.4.

D90/D50 ratio of secondary particles of the suspension of the inventionmay be comprised between 1.2 and 2.5.

Primary particles have an average size D50 comprised between 100 and 300nm, preferably between 100 and 250 nm.

Primary particles have an average size D50 with a standard deviationcomprised between 10 and 20% of the value of said average size of saidsecondary particles.

The primary particles of the suspension preferably provide a specificsurface area (B.E.T.) comprised between 3 and 15 m²/g, more preferablycomprised between 4 and 12 m²/g, particularly between 3 and 10 m²/g.

The liquid phase of the suspensions according to the invention may be ofvarious nature, such as water, water/water-miscible solvent mixture, oran 2.0 organic solvent. The liquid phase of the suspensions may also bean organic solvent mixture.

As an example of a solvent that may be used for a water/water-misciblesolvent mixture, mention may be made of alcohols such as methanol orethanol, glycols such as ethylene glycol, acetate derivatives ofglycols, such as ethylene glycol monoacetate, or polyols.

As an example of an organic solvent, mention may be made of aliphatichydrocarbons such as hexane, heptane, octane or nonane, inertcycloaliphatic hydrocarbons such as cyclohexane, cyclopentane orcycloheptane, aromatic hydrocarbons such as benzene, toluene,ethylbenzene, xylenes, or liquid naphthenes. Also suitable are petroleumfractions of the Isopar or Solvesso type (trade marks registered by thecompany Exxon), in particular Solvesso 100 which contains essentially amixture of methylethylbenzene and trimethylbenzene, Solvesso 150 whichcontains a mixture of alkylbenzenes, in particular of dimethylbenzeneand of tetramethylbenzene, and Isopar which contains essentially C11 andC12 isoparaffinic and cycloparaffinic hydrocarbons. Other types ofpetroleum fractions that may also be mentioned include those ofPetrolink® type from the company Petrolink or of Isane® type from thecompany Total.

Chlorinated hydrocarbons, such as chlorobenzene, dichlorobenzene orchlorotoluene, can also be used as organic solvent. Aliphatic andcycloaliphatic ethers or ketones, for instance diisopropyl ether,dibutyl ether, methyl ethyl ketone, methyl isobutyl ketone, diisobutylketone or mesityl oxide, can be envisaged.

Esters can be used, such as those derived from the reaction of acidswith C1 to C8 alcohols, and in particular palmitates of secondaryalcohols such as isopropanol. By way of example, mention may be made ofbutyl acetate.

Of course, the liquid phase can be based on a mixture of two or morehydrocarbons or compounds of the type described above.

The suspensions of the invention have an overall oxide content, i.e.cerium oxide content, which can vary within wide limits and which can,for example, be comprised between 1 and 40%, in particular between 5 and30% by mass of oxide.

Similarly, the pH of these suspensions can be within a broad range.Thus, the pH of the suspensions derived from the preparation processesthat will be described below is generally between 2 and 6, moreparticularly between 2 and 5, the suspensions remaining stable withinthe meaning given here below, within this pH range. However thestability can be improved either within these ranges of pH either beyondthe value of 5 or 6 in a known manner, by addition to the suspension ofcompounds such as anionic or zwitterionic polymers or molecules. Ascompounds of this kind one can mention those compounds obtained bypolymerizing at least one monomer chosen among the following: anethylenically unsaturated, linear or branched, aliphatic, cyclic oraromatic monocarboxylic or polycarboxylic acid or anhydride. Polyacrylicacid or citric acid may be mentioned as examples.

Finally, it will be noted that the suspensions of the invention arestable. This is intended to mean that no formation of a settling cake isobserved on these suspensions before several days, for example at least8 days. Furthermore, the settling cake, if it forms, can be resuspendedby simple agitation.

Suspensions of the present invention may be obtained according toseveral processes.

Advantageously, the suspension of the invention can be preparedaccording to a process as defined in which one of the importantparameter is precipitation of cerium III salt and cerium IV salt; thecerium IV/total cerium molar ratio before precipitation being comprisedbetween 1/10000 and 1/500000. The cerium IV/total cerium molar ratiobefore precipitation may preferably be comprised between 1/50000 and1/300000.

The first step (a) of the process above therefore consists in treating asolution comprising at least a cerium III salt, a cerium IV salt and abase, under an inert atmosphere, whereby a precipitate is obtained; thecerium IV/total cerium molar ratio before precipitation is comprisedbetween 1/10000 and 1/500000.

As cerium III salts, use may more particularly be made of cerium IIInitrate, chloride, sulfate or carbonate, and also mixtures of thesesalts, such as mixed nitrates/chlorides.

Cerium IV salt may be for example cerium IV nitrate, cerium ammoniumnitrate, cerium ammonium sulfate, and cerium IV sulfate.

It is notably possible in said step a) to provide a starting solution ofa cerium III salt, optionally comprising a cerium IV salt. This solutionmay be then treated with a solution comprising a base and optionally acerium IV salt, under an inert atmosphere, whereby a precipitate isobtained. It is also possible to directly mix cerium III salt, cerium IVsalt and a base. It is notably possible to provide a starting solutionof a cerium III salt and treat this solution with a solution comprisinga base and a cerium IV salt, under an inert atmosphere, whereby aprecipitate is obtained.

In the known lamer, this starting solution of a cerium III salt,optionally comprising a cerium IV salt, should have the acidity suitablefor the cerium to be entirely present in solution.

The starting solution can be degassed beforehand by bubbling with aninert gas. The term “inert gas” or “inert atmosphere” is intended tomean, for the present description, an atmosphere or a gas free ofoxygen, it being possible for the gas to be, for example, nitrogen orargon.

Products of the hydroxide type can in particular be used as base.Mention may be made of alkali metal or alkaline-earth metal hydroxidesand aqueous ammonia. Secondary, tertiary or quaternary amines can alsobe used. However, the amines and the aqueous ammonia may be preferredsince they reduce the risks of pollution by alkali metal cations oralkaline-earth metal cations.

The base can also be degassed beforehand by bubbling with an inert gas.

The amount of ammonia, expressed by the NH₃/Ce molar ratio, ispreferably comprised between 8 and 30.

To perform the reaction, the bringing into contact can be carried out inany order of introducing the reactants. However, it is preferable tointroduce the starting solution into a medium containing the base.

This step should be carried out under an inert atmosphere, either in aclosed reactor or in a semi-closed reactor with sweeping with the inertgas. The bringing into contact is generally carried out in a stirredreactor.

Finally, this step is generally carried out at a temperature comprisedbetween 20° C. and 25° C., or a temperature of at most 50° C.

It is notably possible to first prepare a cerium nitrate solutioncomprising a cerium III salts and then prepare a base solutioncomprising a cerium IV salt. This second solution may be then degassedby bubbling with an inert gas. Cerium nitrate solution may be then mixedwith the base solution comprising comprising a cerium IV salt.

The second step (b) of the process is a thermal treatment of thereaction medium obtained at the end of the preceding step.

This treatment consists in heating the medium and in maintaining it at atemperature which is generally at most 95° C., and more particularlybetween 60° C. and 95° C.

The duration of this treatment can be between a few hours and severaltens of hours.

This treatment is also carried out under an inert atmosphere, thedescription with respect to this atmosphere for the second step beingapplied similarly here. Similarly the treatment is carried out in astirred reactor.

According to one characteristic of the process of the invention, atleast one of steps (a) and (b) should be carried out in the presence ofnitrate ions. In general, the nitrate ions are provided by the additionof nitric acid, more particularly in step (a), during the preparation ofthe cerium III solution.

The amount of nitrate ions, expressed by the NO₃ ⁻/Ce₃ ⁺ molar ratio, isgenerally comprised between 1/6 and 5/1.

The last step of the process, step (c), in fact comprises two successiveoperations which can be carried out in any order. These operations are,firstly, an acidification and, secondly, a wash.

These operations will be described more specifically below, for the caseof a series in which acidification is followed by washing.

The acidification is generally carried out, after cooling of the mediumobtained at the end of step (b), by the addition of an acid.

Any inorganic or organic acid can be used. Nitric acid is moreparticularly used.

The amount of acid added is such that the pH of the medium afteracidification is between 1 and 5.

This operation can be carried out under air; it is no longer necessaryto perform the procedure under an inert atmosphere at this stage of theprocess.

The acidification is followed by washing, the aim of which is to removefrom the suspension the soluble species, essentially salts.

The washing can be carried out in various ways with or withoutsolid/liquid separation.

It can thus be carried out by separating the solid particles from theliquid phase, for example by frontal filtration, settling out orcentrifugation. The solid obtained is then re-suspended in an aqueousphase. The process can also be carried out by tangential filtration.

This washing can be optionally repeated if necessary, for example untila given conductivity of the suspension is obtained, whereby theconductivity measures the amount of impurities present in thissuspension.

As indicated above, the order of the operations can be reversed comparedwith that which has just been described. Thus, at the end of step (b),and, here again, generally after cooling of the medium obtained, washingcan then carried out in the manner described above. At the end of thewashing, the acidification of the medium obtained is then carried out.

At the end of the steps which have been described, it is possible totreat the suspension which has been obtained in a known deagglomerationapparatus such as an apparatus of ultrasonic treatment, of double impactjet treatment or a wet milling apparatus.

A suspension according to the invention is obtained at the end of step(c).

In the case of a suspension partially or completely in an organicsolvent medium other than water, this suspension can be prepared, in amanner known per se, from an aqueous suspension as obtained by means ofthe process which has just been described and by bringing into contactwith the organic solvent.

At this stage, it may be advantageous to add to the organic phase apromoter agent whose function is to accelerate the transfer of theparticles from the aqueous phase to the organic phase and to improve thestability of the organic suspensions obtained.

As a promoter agent, use may be made of compounds comprising an alcoholfunction, and most particularly linear or branched aliphatic alcoholshaving from 6 to 12 carbon atoms. As specific examples, mention may bemade of 2-ethylhexanol, decanol, dodecanol, or mixtures thereof.

The bringing into contact can be carried out at ambient temperature, forexample approximately 20° C., but also at a higher temperature, forexample in a range of from 60° C. to 150° C.

The separation between the aqueous and organic phases is carried out,for example, by distillation, by settling out or by centrifugationdepending on the nature of the organic solvent.

The invention also relates to a redispersible powder of cerium oxideparticles. It is a characteristic of this powder that, afterintroduction into a liquid phase and redispersion in a liquid phase, itproduces a suspension according to the invention as described above. Theredispersion is carried out by simple agitation of the powder in theliquid phase.

This powder can be obtained from a suspension according to the inventionby drying and then calcination at a temperature which may be, forexample, at most 300° C., and in particular between 100° C. and 200° C.,over a period which can range between a few minutes and a few hours.

The invention also relates to a suspension for polishing, comprisingeither a suspension as described above, or a suspension as obtained bymeans of the processes described above, or else a suspension obtainedafter redispersion of a powder according to the invention. Thissuspension can be used for polishing glass, for example in thecrystal-making or mirror industry, flat glass, television screens orspectacles, or else for polishing ceramics or other materials ofvitreous type. This suspension can also be used most particularly forCMP-type polishing in the electronics industry and therefore forpolishing metal substrates which go to make up microprocessors, but alsofor polishing insulating layers of these same microprocessors, thesuspension of the invention being particularly suitable for thepolishing of said layers. These layers are generally made of silica(doped silica, porous silica).

In general, such suspensions comprise, in addition to the compound withabrasive property, such as the cerium oxide particles, additives such asa dispersing agent or an oxidant.

The present invention also concerns a method of removing a portion of asubstrate, notably in a CMP operation, comprising at least the followingsteps:

-   -   Providing at least a suspension of the invention,    -   Contacting at least the suspension and the substrate to be        polished, and    -   Performing the polishing on the substrate.

As other applications of the suspensions of the invention, mention maybe made of catalysis, in particular for automobile post-combustion; inthis case, the suspensions are used in the preparation of catalysts. Thesuspensions can also be used for their anti-UV properties, for examplein the preparation of films of polymers (of the acrylic or polycarbonatetype, for example), of paints, of papers or of cosmetic compositions, inparticular in the preparation of anti-UV creams.

The following examples are included to illustrate embodiments of theinvention. Needless to say, the invention is not limited to describedexamples.

EXPERIMENTAL PART COMPARATIVE EXAMPLE 1

A dilute cerium nitrate solution was prepared by adding 13.8 kg of a 3Mtrivalent cerium nitrate solution, 2.2 kg of 68% HNO₃ solution, 0.4 kgof deionized water and cerium nitrate (IV) equivalent with 1/1250 ofcerium IV/total cerium molar ratio. This solution was loaded into asemi-closed 201 vessel and then degassed with agitation and withnitrogen bubbling.

A dilute aqueous ammonia solution is prepared by adding 80 kg ofdeionized water and a solution of 9.3 kg of 25% aqueous ammonia. Thissolution is loaded into a semi-closed 1001 jacketed reactor and thensubjected to agitation and nitrogen bubbling.

The diluted cerium nitrate solution is then added, at ambienttemperature, to the dilute aqueous ammonia solution, with the sameagitation and under nitrogen sweeping. The temperature of the reactionmixture is then increased to 80° C. and then maintained at thistemperature. At the end of this heat treatment, the reaction mixture isleft to cool and was acidified to pH 2 by adding 68% HNO₃.

The reaction mixture was cooled and acidified at pH 2 with 68% HNO₃. Thereaction mixture was filtrated and washed with deionized water. Thewashing was repeated when the conductivity of washing solution was less0.04 mS/cm. The suspension obtained finally was adjusted at 30% of CeO₂.

The BET specific surface area determined by nitrogen adsorption was 13m²/g.

The suspension was observed by TEM. The primary particles weremonodispersed and the size was about 94 nm. For approximately 150particles representative of the suspension, each of particles werecounted and measured. The average particle size was 94 nm and standarddeviation was 21 nm corresponding to 22% of average particle size.

The secondary particle size was measured at relative refractive index1.7 of CeO₂ in the water by laser particle sizer (Horiba LA-910). Themedian size D50 was 96 nm and standard deviation was 36 nm correspondingto 26% of average particle size. The D10, D50 and D90 were 82, 96 and122 nm, respectively. The calculated dispersion σ/m and D90/D50 were0.21 and 1.27 respectively.

EXAMPLE 1

A cerium nitrate solution was prepared by mixing 13.8 kg of 3M trivalentcerium nitrate, 2.2 kg of 68% HNO₃ and 0.4 kg of deionized water. Thissolution was put into 20 L semi-closed vessel.

The ammonia aqueous solution was prepared by adding 15.5 kg of 25%ammonia water and 73 kg of deionized water. Subsequently cerium nitrate(IV) equivalent with 1/80000 of cerium IV/total cerium molar ratio wasadded. This solution was put into 100 L semi-closed reactor jacketed,and bubbled by N₂ gas with the agitation for 1 hour.

The above described cerium nitrate solution was mixed with the ammoniaaqueous solution in approximately 30 min in the same conditions ofagitation and N₂ bubbling.

The temperature of reaction mixture was heated up to 80° C. inapproximately 1 hours and maintained for approximately 10 hours at thesame conditions of agitation without N₂ bubbling.

The reaction mixture was cooled and acidified at pH 2 with 68% HNO₃. Thereaction mixture was filtrated and washed with deionized water. Thewashing was repeated when the conductivity of washing solution was less0.04 mS/cm. The suspension obtained finally was deagglomerated andadjusted at 30% of CeO₂.

The BET specific surface area determined by nitrogen adsorption was 10m²/g.

The suspension was observed by TEM. The primary particles weremonodispersed and the size was about 110 nm. For approximately 150particles representative of the suspension, each of particles werecounted and measured. The average particle size was 110 nm and standarddeviation was 25 nm corresponding to 22% of average particle size. TEMpicture is reported in FIG. 1.

The secondary particle size was measured at relative refractive index1.7 of CeO₂ in the water by laser particle sizer (Horiba LA-910). Themedian size D50 was 113 nm and standard deviation was 17 nmcorresponding to 15% of average particle size. The D10, D50 and D90 were96, 113 and 139 nm, respectively. The calculated dispersion σ/m andD90/D50 were 0.19 and 1.23 respectively.

EXAMPLE 2

A cerium nitrate solution was prepared by mixing 13.8 kg of 3M trivalentcerium nitrate, 1.3 kg of 68% HNO₃ and 1.0 kg of deionized water. Thissolution was put into 20 L semi-closed vessel.

The ammonia aqueous solution was prepared by adding 15.5 kg of 25%ammonia water and 73 kg of deionized water. Subsequently cerium nitrate(IV) equivalent with 1/80000 of cerium IV/total cerium molar ratio wasadded. This solution was put into 100 L semi-closed reactor jacketed,and bubbled by N₂ gas with the agitation for 1 hour.

The above described cerium nitrate solution was mixed with the ammonia.aqueous solution in approximately 30 min in the same conditions ofagitation and N₂ bubbling.

The temperature of reaction mixture was heated up to 80° C. inapproximately 1 hours and maintained for approximately 10 hours at thesame conditions of agitation without N₂ bubbling.

The reaction mixture was cooled and acidified at pH 2 with 68% HNO₃, Thereaction mixture was filtrated and washed with deionized water. Thewashing was repeated when the conductivity of washing solution was less0.04 mS/cm. The suspension obtained finally was deagglomerated andadjusted at 30% of CeO₂.

The BET specific surface area determined by nitrogen adsorption was 10m²/g.

The suspension was observed by TEM. The primary particles weremonodispersed and the size was about 128 nm. For approximately 150particles representative of the suspension, each of particles werecounted and measured. The average particle size was 128 nm and standarddeviation was 17 nun corresponding to 13% of average particle size.

The secondary particle size was measured at relative refractive index1.7 of CeO₂ in the water by laser particle sizer (Horiba LA-910). Themedian size D50 was 128 nm and standard deviation was 29 nmcorresponding to 23% of average particle size. The D10, D50 and D90 were103, 128 and 170 nm, respectively. The calculated dispersion σ/m andD90/D50 were 0.26 and 1.33 respectively.

EXAMPLE 3

A cerium nitrate solution was prepared by mixing 13.5 kg of 3M trivalentcerium nitrate, 2.2 kg of 68% HNO₃ and 0.7 kg of deionized water. Thissolution was put into 20L semi-closed vessel. The ammonia aqueoussolution was prepared by adding 15.5 kg of 25% ammonia water and 79 kgof deionized water. Subsequently cerium nitrate (IV) equivalent with1/80000 of cerium IV/total cerium molar ratio was added. This solutionwas put into 100 L semi-closed reactor jacketed, and bubbled by N₂ gaswith the agitation for 1 hour.

The above described cerium nitrate solution was mixed with the ammoniaaqueous solution in approximately 30 min in the same conditions ofagitation and N₂ bubbling. The temperature of reaction mixture washeated up to 85° C. in approximately 1 hour and maintained forapproximately 10 hours at the same conditions of agitation without N₂bubbling. The reaction mixture was cooled and acidified at pH 2 with 68%HNO₃. The reaction mixture was filtrated and washed with deionizedwater. The washing was repeated when the conductivity of washingsolution was less 0.04 mS/cm. The suspension obtained finally wasdeagglomerated and adjusted at 30% of CeO₂.

The BET specific surface area determined by nitrogen adsorption was 9m²/g.

The suspension was observed by TEM. The primary particles weremonodispersed and the size was about 129 nm. For approximately 150particles representative of the suspension, each of particles werecounted and measured. The average particle size was 129 nm and standarddeviation was 24 nm corresponding to 19% of average particle size.

The secondary particle size was measured at relative refractive index1.7 of CeO₂ in the water by laser particle sizer (Horiba LA-910). Themedian size D50 was 130 nm and standard deviation was 32 nmcorresponding to 25% of average particle size. The D10, D50 and D90 were104, 130 and 178 nm, respectively. The calculated dispersion σ/m andD90/D50 were 0.28 and 1.37 respectively.

EXAMPLE 4

A cerium nitrate solution was prepared by mixing 13.8 kg of 3M trivalentcerium nitrate, 2.2 kg of 68% HNO₃ and 0.4 kg of deionized water. Thissolution was put into 20 L semi-closed vessel.

The ammonia aqueous solution was prepared by adding 15.5 kg of 25%ammonia water and 73 kg of deionized water. Subsequently cerium nitrate(IV) equivalent with 1/140000 of cerium IV/total cerium molar ratio wasadded. This solution was put into 100 L semi-closed reactor jacketed,and bubbled by N₂ gas with the agitation for 1 hour.

The above described cerium nitrate solution was mixed with the ammoniaaqueous solution in approximately 30 min in the same conditions ofagitation and N₂ bubbling.

The temperature of reaction mixture was heated up to 80° C. inapproximately 1 hours and maintained for approximately 15 hours at thesame conditions of agitation without N₂ bubbling.

The reaction mixture was cooled and acidified at pH 2 with 68% HNO₃. Thereaction mixture was filtrated and washed with deionized water. Thewashing was repeated when the conductivity of washing solution was less0.04 mS/cm. The suspension obtained finally was deagglomerated andadjusted at 30% of CeO₂.

The BET specific surface area determined by nitrogen adsorption was 9m²/g.

The suspension was observed by TEM. The primary particles weremonodispersed and the size was about 131 nm For approximately 150particles representative of the suspension, each of particles werecounted and measured. The average particle size was 131 nm and standarddeviation was 14 nm corresponding to 11% of average particle size.

The secondary particle size was measured at relative refractive index1.7 of CeO₂ in the water by laser particle sizer (Horiba LA-910). Themedian size D50 was 129 nm and standard deviation was 32 nmcorresponding to 25% of average particle size. The D10, D50 and D90 were102, 129 and 177 nm, respectively. The calculated dispersion σ/m andD90/D50 were 0.29 and 1.37 respectively.

EXAMPLE 5

A cerium nitrate solution was prepared by mixing 13.5 kg of 3M trivalentcerium nitrate, 2.2 kg of 68% HNO₃ and 0.7 kg of deionized water. Thissolution was put into 20 L semi-closed vessel.

The ammonia aqueous solution was prepared by adding 15.5 kg of 25%ammonia water and 73 kg of deionized water. Subsequently cerium nitrate(IV) equivalent with 1/80000 of cerium IV/total cerium molar ratio wasadded. This solution was put into 100 L semi-closed reactor jacketed,and bubbled by N₂ gas with the agitation for 1 hour.

The above described cerium nitrate solution was mixed with the ammoniaaqueous solution in approximately 30 min in the same conditions ofagitation and N₂ bubbling.

The temperature of reaction mixture was heated up to 85° C. inapproximately 1 hour and maintained for approximately 20 hours at thesame conditions of agitation without N₂ bubbling.

The reaction mixture was cooled and acidified at pH 2 with 68% HNO₃. Thereaction mixture was filtrated and washed with deionized water. Thewashing was repeated when the conductivity of washing solution was less0.04 mS/cm. The suspension obtained finally was deagglomerated andadjusted at 30% of CeO₂.

The BET specific surface area determined by nitrogen adsorption was 8m²/g.

The suspension was observed by TEM. The primary particles weremonodispersed and the size was about 165 nm. For approximately 150particles representative of the suspension, each of particles werecounted and measured. The average particle size was 165 nm and standarddeviation was 25 nm corresponding to 15% of average particle size.

The secondary particle size was measured at relative refractive index1.7 of CeO₂ in the water by laser particle sizer (Horiba LA-910). Themedian size D50 was 137 nm and standard deviation was 36 nmcorresponding to 26% of average particle size. The D10, D50 and D90 were106, 137 and 192 nm, respectively. The calculated dispersion σ/m andD90/D50 were 0.31 and 1.40 respectively.

The properties of secondary particle after deagglomeration for thesuspensions of Examples 1 to 5 and Comparative Example 1 are reported inTable 1.

TABLE 1 SSA D10 D50 D90 Samples (m²/g) (nm) (nm) (nm) D90/D50 σ/mComparative 1 13 82 96 122 1.27 0.21 1 10 96 113 139 1.23 0.19 2 10 103128 170 1.33 0.26 3 9 104 130 178 1.37 0.28 4 9 102 129 177 1.37 0.29 58 106 137 192 1.40 0.31 σ/m = (D90 − D10)/(2 * D50)

EXAMPLE 6 Polishing Application

The suspensions of Examples 1 to 5 and Comparative Example 1 were testedin a polishing application with the following polishing conditions:

Polishing tool: LM15

Pad: SFMA-soft polyurethane pad

Wafer: Quartz glass with a coefficient for RR calc=45.564

Slurry concentration: 1 wt %

Load: 10 kg

Pad speed: 80 rpm/Drivers speed: 40 rpm/Agitation speed: 400 rpm

Pump flow: 500 mL/min

Polishing duration: 30 mins (one shot)

Polishing performances are reported in Table 2.

TABLE 2 Removal rate Samples (μm/min) Comparative 1 0.037 1 0.066 20.090 3 0.102 4 0.082 5 0.137

Removal rate was measured as follows: before polishing, the weight of aSiO₂ quartz substrate was measured by a balance. After polishing, theSiO₂ quartz substrate was washed and dried. The weight of SiO₂ quartzsubstrate after polishing was measured. The removal rate was calculatedby “weight loss before and after polishing”, “polishing duration” and“the density and the area of SiO₂ quartz substrate”. The followingformula was used for calculation.

${RR} = \frac{\left( {m_{before} - m_{after}} \right)*10^{3}}{A_{surf}*\rho_{quartz}*t_{\exp}}$

t_(exp)=30 min, ρ_(quartz)=2.2 g/cm³, A_(surf)=100 cm²

RR: removal rate (m/min)

m: weight of SiO₂ quartz substrate (g)

A: surface area of SiO₂ quartz substrate (cm²)

ρ: density of SiO₂ quartz g/cm³)

t: duration time (min)

The suspension of cerium oxide particles in a liquid phase according tothe present invention (Examples 1-5) permits to reach higher removalrates in the polishing application in comparison with suspensions of theprior art (Comp. Example 1), without observing scratches on the treatedsubstrate.

1. A suspension of cerium oxide particles in a liquid phase, whereinsaid particles comprise secondary particles comprising primaryparticles, and wherein: the secondary particles have an average size D50of between 105 and 1000 nm, with a standard deviation of between 10 and50% of the value average size of the secondary particles; and theprimary particles have an average size D50 of between 100 and 300 nm,with a standard deviation of between 10 and 30% of the average size ofthe primary particles.
 2. The suspension according to claim 1, whereinthe secondary particles have an average size D50 of between 110 and 800nm.
 3. The suspension according to claim 1, wherein the primaryparticles have an average size D50 of between 100 and 250 nm.
 4. Thesuspension according to claim 1, wherein the secondary particles have adispersion index of at most 0.6.
 5. The suspension according to claim 4,wherein the secondary particles have a dispersion index of between 0.22and 0.6.
 6. The suspension according to claim 5, wherein the secondaryparticles have a dispersion index of between 0.26 and 0.6.
 7. Thesuspension according to claim 1, wherein the primary particles have aBET specific surface area of between 3 and 15 m²/g.
 8. The suspensionaccording to claim 7, wherein the primary particles have a BET specificsurface area of between 3 and 10 m²/g.
 9. The suspension according toclaim 1, wherein the liquid phase is at least one liquid selected fromthe group consisting of: water, water/water-miscible solvent mixture,and an organic solvent.
 10. A process for preparing a cerium oxidesuspension, the process comprising: (a) treating a solution comprisingat least a cerium III salt, a cerium IV salt and a base, under an inertatmosphere, to form a precipitate in solution; wherein the ceriumIV/total cerium molar ratio before precipitation is between 1/10000 and1/500000; (b) thermally treating the precipitate in solution under aninert atmosphere to form a thermally treated medium, wherein at leastone of steps (a) or (b) is carried out in the presence of nitrate ions;and (c) acidifying and washing the thermally treated medium, whereby thesuspension is obtained.
 11. The process according to claim 10, whereinthe cerium IV/total cerium molar ratio before precipitation is between1/50000 and 1/300000.
 12. The process according to claim 10, wherein thecerium IV is at least one compound selected from the group consistingof: cerium IV nitrate, cerium ammonium nitrate, cerium ammonium sulfate,and cerium IV sulfate.
 13. The process according to claim 10, whereinthe amount of nitrate ions, expressed by the NO₃ ⁻/Ce₃ ⁺ molar ratio, isbetween 1/6 and 5/1.
 14. The process according to claim 10, wherein stepa) comprises treating a solution of a cerium III salt with a solutioncomprising a base and a cerium IV salt, under an inert atmosphere, toform a precipitate in solution.
 15. A method of removing a portion of asubstrate, the method comprising: polishing the substrate with asuspension according to claim 1.